LTM4650 datasheet(41/138 Pages) AD

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LTM4664A

Rev. 0

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connecting EXTVCC to INTVCC. Using the EXTVCC allows

the INTVCC power to be derived from a high efficiency

external source such as a switching regulator output.

EXTVCC can provide power to the internal 3.3V linear reg-

ulator even when VINS3 is not present, which allows the

LTM4664A PSM to be initialized and programmed even

without main power being applied.

The INTVCC regulator is powered from the VINS3_C1 pin,

the power through the IC is equal to VINS3_C1 • IINTVCC. The

gate charge current is dependent on operating frequency.

The INTVCC regulator can supply up to 100mA, and the

typical INTVCC current for the LTM4664A PSM is ~50mA. A

12V input voltage would equate to a difference of 7V drop

across the internal controller, when multiplied by 50mA

equals a 350mW power loss. This loss can be eliminated

by providing an external 5V bias on the EXTVCC pin.

Do not tie INTVCC on the LTM4664A PSM to an external

supply because INTVCC will attempt to pull the external

supply high and hit current limit, significantly increasing

the die temperature.

OUTPUT CURRENT SENSING AND SUB MILLIOHM

DCR CURRENT SENSING

The LTM4664A PSM channels use a unique sub-mil-

liohm inductor current sensing technique that provides

a high level signal to noise ratio while sensing very low

signals in current mode operation. This enables higher

conversion efficiencies with the use of the internal

sub-milliohm inductors in heavy load applications. The

current limit threshold can be accurately set with the

MFR_PWM_MODE[7] for high and low range. The low

range setting MFR_PWM_MODE [7] = 0 should be used

(see page103).

TheinternalDCRsensingnetwork,thuscurrentlimitarecalcu-

lated based on the DCR of the inductor at room temperature.

The DCR of the inductor has a large temperature coefficient,

approximately 3900ppm/°C. The temperature coefficient of

the inductor is written to the MFR_IOUT_CAL_GAIN_TC reg-

ister. The external temperature is sensed near the inductor

and used to modify the internal current limit circuit to main-

tain an essentially constant current limit with temperature.

The current sensed is then digitized by the LTM4664A PSM

telemetry ADC with an input range of ±128mV, a noise floor

of 7µVRMS, and a peak-peak noise of approximately 46.5µV.

The LTM4664A PSM computes the inductor current using

the DCR value stored in the IOUT_CAL_GAIN command and

the temperature coefficient stored in command MFR_IOUT_

CAL_GAIN_TC. The resulting current value is returned by the

READ_IOUT command.

INPUT CURRENT SENSING

To sense total input current consumed by the LTM4664A's

25A/30A two power stages , a sense resistor is placed

between the supply voltage and VINS3 path. The IIN+ and

IIN– pins are connected to the sense resistor. The filtered

voltage is amplified by the internal high side current

sense amplifier and digitized by the LTM4664A’s PSM

telemetry ADC. The input current sense amplifier has

three gain settings of 2x, 4x, and 8x set by the bit[6:5] of

the MFR_PWM_CONFIG command. The maximum input

sense voltage for the three gain settings is 50mV, 20mV,

and 5mV respectively. The LTM4664A PSM computes the

input current using the internal RSENSE value stored in the

IIN_CAL_GAIN command. The resulting measured power

stage current is returned by the READ_IIN command.

The LTM4664A uses a 1Ω resistor to measure the chip

supply current being consumed by the LTM4664A PSM

Controller. This value is returned by the MFR_READ_ICHIP

command. The chip current is calculated by using the 1Ω

value stored in the MFR_RVIN command. Refer to the

subsection titled Input Current Sense Amplifier in the

Dual 25A/30A PSM Applications Information section for

further details.

PolyPhase LOAD SHARING

Multiple LTM4664As can be arrayed in order to provide

a balanced load-share solution by bussing the necessary

pins. Figure 48 illustrates a 4-Phase design sharing con-

nections required for load sharing.

If an external oscillator is not provided, the SYNC pin

should only be enabled on one of the LTM4664A's

PSM Channels. The other(s) should be programmed to

disable SYNC using bit 4 of MFR_CONFIG_ALL. If an

external oscillator is present, the chip with the SYNC

DUAL 25A/30A PSM OPERATION

LTM4650 Datasheet(PDF) 41 Page - Analog Devices

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